Flexible joints having an annular elastomeric flex element mounting an extension to a housing are used for reducing motion-induced stresses between floating offshore facilities and risers and tendons depending from the floating offshore facilities. Typically the flex element consists of alternating spherical shims of metal, or other stiff material, and layers of elastomeric material. Such a flex element is capable of providing a free angular displacement of about ±15 degrees or more while supporting an axial tension proportional to the size of the flex element. Typically the size of the flex element has been selected to handle the desired load upon the riser or tendon, and flex elements have been manufactured and stocked in various sizes for handling various standard sizes of risers or tendons.
Risers are used for transfer of production fluids from the sea floor up to a deck of a floating offshore vessel, and for transfer of the production fluid off the vessel to one or more export lines. The loads impressed by the riser on a flex element typically consist of tension in the riser, angular displacement and rotation of the riser, internal pressure in the production fluid, and increased temperature from the production fluid. Thus, the internal pressure in the production fluid, and increased temperature from the production fluid, may make the selection of a flex element for a riser more difficult than the selection of a flex element for a tendon.
For various applications, flexible pipe joints have incorporated more than one flex element in a common housing. For example, a double-ended flexible pipe joint for a riser has a first flex element in the housing for mounting a first extension pipe to the housing, and a second flex element in the housing for mounting a second extension pipe to the housing. The two extension pipes extend in opposite directions from the common housing. In this fashion, the double-ended flexible pipe joint can accommodate twice the angular displacement than can be tolerated by a single-ended flexible pipe joint having a single flex element. The angular displacement is divided between the two flex elements in the double-ended flexible pipe joint, but each of the two flex elements carries the same full tension of the riser. Examples of such double-ended flexible pipe joint are found in Herbert et al. U.S. Pat. No. 3,680,895 issued Aug. 1, 1972; Herbert et al. U.S. Pat. No. 4,068,864 issued Jan. 17, 1978 (see FIG. 4); and Whightsil, Sr. et al. U.S. Pat. No. 5,133,578 issued Jul. 28, 1992.
Flexible pipe joints have incorporated more than one flex element in a common housing so that two flex elements are subjected to the same angular displacement yet only one of these two flex elements carries the tensile load upon the flexible pipe joint. Such an arrangement may reduce pressure from production fluid on each flex element and provide both a primary and a backup sealing mechanism for containing the pressurized production fluid within the pipe joint. However, the flex elements in these concepts need to be pre-compressed for proper functioning; a fact that reduces the usable life of the flex elements. Thus, these designs make an inefficient use of the two flex elements both to carry the axial load upon the pipe and to seal the pressure. Examples of such flexible pipe joints are found in Schwemmer U.S. Pat. No. 4,183,556 issued Jan. 15, 1980, and Ohrt U.S. Pat. No. 4,068,868 issued Jan. 17, 1978.
Flexible pipe joints having more than one flex element in a common housing have also been used to provide a coaxial dual-lumen riser. For example, as described in Peppel et al. U.S. Pat. No. 4,784,410 issued Nov. 15, 1988 and in Peppel et al. U.S. Pat. No. 4,984,827 issued Jan. 15, 1991, within a flexible pipe joint, at least two concentric and separate passages are formed for conveying fluids through the flexible pipe joint. The passages are said to remain separate and essentially unchanging in cross-section throughout the entire range of pivotal motion of the flexible pipe joint. However, in each of the several versions disclosed in Peppel et al. U.S. Pat. Nos. 4,784,410 and 4,984,827, the flex elements in the common housing are forced by connecting rings to react in parallel to a common angular displacement, thus assuring the cross sections of the passages to remain essentially unchanged during rotation of the joint, but are uncoupled axially such that tensile load upon the flexible joint is carried by one flex element only.